U.S. patent application number 16/342859 was filed with the patent office on 2019-08-15 for charged particle beam steering arrangement.
The applicant listed for this patent is RELIANCE PRECISION LIMITED. Invention is credited to Nigel CROSLAND, Phil HOYLE, Ian LAIDLER, Andrew MCCLELLAND.
Application Number | 20190252152 16/342859 |
Document ID | / |
Family ID | 57680782 |
Filed Date | 2019-08-15 |
United States Patent
Application |
20190252152 |
Kind Code |
A1 |
CROSLAND; Nigel ; et
al. |
August 15, 2019 |
CHARGED PARTICLE BEAM STEERING ARRANGEMENT
Abstract
A method of forming a product using additive layer manufacture
is provided. The method comprises forming the product as a series
of layers, each layer being formed by fusing powder deposited as a
powder bed by scanning the powder bed using a charged particle beam
to form a desired layer shape. For each layer, the powder is fused
by melting successive areas of the powder bed by scanning the
charged particle beam using a combination of a relatively
long-range deflector and a relatively short-range deflector,
wherein the relatively long-range deflector deflects the charged
particle beam over a larger deflection angle than the short-range
deflector. Also provided are a corresponding charged particle
optical assembly, and an additive layer manufacturing
apparatus.
Inventors: |
CROSLAND; Nigel; (Linton,
GB) ; MCCLELLAND; Andrew; (Cambridge, GB) ;
HOYLE; Phil; (Chesterton, Cambridge, GB) ; LAIDLER;
Ian; (Shelley, Huddersfield, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
RELIANCE PRECISION LIMITED |
Huddersfield, West Yorkshire |
|
GB |
|
|
Family ID: |
57680782 |
Appl. No.: |
16/342859 |
Filed: |
October 18, 2017 |
PCT Filed: |
October 18, 2017 |
PCT NO: |
PCT/EP2017/076636 |
371 Date: |
April 17, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 37/1475 20130101;
B22F 3/1055 20130101; Y02P 10/295 20151101; H01J 2237/30483
20130101; H01J 37/302 20130101; B22F 2003/1057 20130101; Y02P 10/25
20151101; B22F 2003/1056 20130101; H01J 37/3023 20130101; H01J
37/1472 20130101; H01J 37/305 20130101 |
International
Class: |
H01J 37/305 20060101
H01J037/305; H01J 37/147 20060101 H01J037/147; H01J 37/302 20060101
H01J037/302; B22F 3/105 20060101 B22F003/105 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 19, 2016 |
GB |
1617693.5 |
Claims
1. A method of forming a product using additive layer manufacture,
comprising: forming the product as a series of layers, each layer
being formed by fusing powder deposited as a powder bed by scanning
the powder bed using a charged particle beam to form a desired
layer shape; wherein, for each layer, the powder is fused by
melting successive areas of the powder bed by scanning the charged
particle beam using a combination of a relatively long-range
deflector and a relatively short-range deflector wherein the
relatively long-range deflector deflects the charged particle beam
over a larger deflection angle than the short-range deflector,
wherein the method comprises using the relatively long-range
deflector to set a base position of the charged particle beam on
the powder bed and using the relatively short-range deflector to
scan the charged particle beam around the base position set by the
relatively long-range deflector, the method further comprising
repeated steps of using the relatively long-range deflector to set
different base positions of the charged particle beam on the powder
bed and using the relatively short-range deflector to scan the
charged particle beam around each of the different base positions
set by the relatively long-range deflector to trace a series of
predetermined shapes on the powder bed, wherein the predetermined
shapes combine to create the desired layer shape, the method
further comprising repeatedly using the relatively short-range
deflector to scan the charged particle beam to trace the same
predetermined shape at each base position on the powder bed, and
controlling the current of the charged particle beam and the scan
speed of the relatively short-range deflector to raise and maintain
the temperature of the powder bed evenly within the predetermined
shape.
2. The method of claim 1, comprising using the relatively
short-range deflector to trace a series of predetermined shapes,
the majority of which have a common size and shape and tessellate
to form part of the desired layer shape.
3. The method of claim 2, comprising using the relatively
long-range deflector to set an array of base positions of the
charged particle beam on the powder bed, with the predetermined
shapes scanned by the charged particle beam about each base
position tessellating without leaving gaps there-between thereby
forming a part of the desired shape.
4. The method of claim 2, comprising using the relatively
long-range deflector to scan the charged particle beam at a
relatively slow speed and using the relatively short-range
deflector to scan the charged particle beam at a relatively fast
speed.
5. The method of claim 4, wherein the long-range deflector is an
electromagnetic deflector comprising a Helmholtz coil with more
than 25 turns per coil and the short-range deflector is an
electromagnetic deflector comprising a Helmholtz coil with fewer
than 5 turns per coil.
6. The method of claim 1, comprising: setting the relatively
long-range deflector to position the charged particle beam at the
base position of the charged particle beam on the powder bed,
maintaining the setting of the relatively long-range deflector
while varying the setting of the relatively short-range deflector
to scan the charged particle beam around the base position set by
the relatively long-range deflector; and further steps of: changing
the setting of the relatively long-range deflector to position the
charged particle beam at a different base position of the charged
particle beam on the powder bed, maintaining the setting of the
relatively long-range deflector while varying the setting of the
relatively short-range deflector to scan the charged particle beam
around the different base position set by the relatively long-range
deflector.
7. The method of claim 1, comprising: varying the relatively
long-range deflector to scan the charged particle beam through a
series of base positions of the charged particle beam on the powder
bed while varying the setting of the relatively short-range
deflector to scan the charged particle beam around the base
positions set by the relatively long-range deflector.
8. A charged particle optical assembly for use in additive layer
manufacture, comprising: a charged particle source; beam forming
apparatus operable to form a beam of charged particles from the
charged particles provided by the charged particle source that
travels along a direction of propagation; and beam steering
apparatus; wherein the beam steering apparatus comprises a
long-range deflector operable to deflect the charged particle beam
over a relatively large deflection angle and to set a base position
of the charged particle beam on the powder bed, and short-range
deflector operable to deflect the charged particle beam over only a
relatively small deflection angle and to scan the charged particle
beam around the base position set by the relative long-range
deflector; wherein the relatively long-range deflector configured
to be used repeatedly to set different base positions of the
charged particle beam on the powder bed and the relatively
short-range deflector configured to be used to scan the charged
particle beam around each of the different base positions set by
the relatively long-range deflector to trace a series of
predetermined shapes on the powder bed, wherein the predetermined
shapes combine to create the desired layer shape, wherein the
relatively small-range deflector is configured to scan the charged
particle beam to trace the same predetermined shape at each base
position on the powder bed, and the current of the charged particle
beam is controlled and the scan speed of the relatively short-range
deflector is controlled to raise and maintain the temperature of
the powder bed evenly within the predetermined shape.
9. The charged particle optical assembly of claim 8, wherein the
long-range deflector is arranged to cause the charged particle beam
to deflect transversely to the direction of propagation.
10. The charged particle optical assembly of claim 9, wherein the
long-range deflector comprises first and second deflectors arranged
to act orthogonally with respect to each other and to the direction
of propagation.
11. The charged particle optical assembly of claim 8, wherein the
short-range deflector is arranged to cause the charged particle
beam to deflect transversely to the direction of propagation.
12. The charged particle optical assembly of claim 11, wherein the
short-range deflector comprises first and second deflectors
arranged to act orthogonally with respect to each other and to the
direction of propagation.
13. The charged particle optical assembly of claim 12 when
dependent upon claim 10, wherein the first deflectors of the
long-range deflector and the short-range deflector are arranged to
deflect the charged particle beam in a common direction, and the
second deflectors of the long-range deflector and the short-range
deflector are arranged to deflect the charged particle beam in a
common direction.
14. The charged particle optical assembly of claim 13, wherein the
first and second deflectors of both the long-range deflector and
the short-range deflector comprise Helmholtz coils with a coil of
wire of other electrical current carrying medium arranged to either
side of the charged particle beam path.
15. The charged particle optical assembly of claim 14, wherein the
coils of the long-range deflector comprise 50 to 100 turns of wire
and/or wherein the coils of the short-range deflector comprise 1 to
5 turns of wire.
16. The charged particle optical assembly of claim 8, wherein the
first and second deflectors of both the long-range deflector and
the short-range deflector comprise electrostatic deflectors that
are arranged to either side of the charged particle beam path.
17. The charged particle optical assembly of claim 8, wherein the
charged particles are electrons and the charged particle source is
an electron source.
18. An additive layer manufacturing apparatus comprising: the
charged particle optical assembly of claim 8; at least one hopper
operable to dispense powder; and a table positioned to receive the
powder dispensed by the at least one hopper in a volume defining a
powder bed for receiving the charged particle beam.
19. The apparatus of claim 18, wherein the long-range deflector is
operable to scan the electron beam over at least half the area of
the powder bed and the short-range deflector is operable to scan
the electron beam over less than half the area of the powder
bed.
20. The additive layer manufacturing apparatus of claim 18, further
comprising a controller programmed to perform a method of forming a
product using additive layer manufacture, the method comprising:
forming the product as a series of layers, each layer being formed
by fusing powder deposited as a powder bed by scanning the powder
bed using a charged particle beam to form a desired layer shape;
wherein, for each layer, the powder is fused by melting successive
areas of the powder bed by scanning the charged particle beam using
a combination of a relatively long-range deflector and a relatively
short-range deflector wherein the relatively long-range deflector
deflects the charged particle beam over a larger deflection angle
than the short-range deflector, wherein the method comprises using
the relatively long-range deflector to set a base position of the
charged particle beam on the powder bed and using the relatively
short-range deflector to scan the charged particle beam around the
base position set by the relatively long-range deflector, the
method further comprising repeated steps of using the relatively
long-range deflector to set different base positions of the charged
particle beam on the powder bed and using the relatively
short-range deflector to scan the charged particle beam around each
of the different base positions set by the relatively long-range
deflector to trace a series of predetermined shapes on the powder
bed, wherein the predetermined shapes combine to create the desired
layer shape, the method further comprising repeatedly using the
relatively short-range deflector to scan the charged particle beam
to trace the same predetermined shape at each base position on the
powder bed, and controlling the current of the charged particle
beam and the scan speed of the relatively short-range deflector to
raise and maintain the temperature of the powder bed evenly within
the predetermined shape.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an additive layer
manufacturing apparatus comprising a novel charged particle beam
steering arrangement and, in particular, to a novel electron beam
steering arrangement. The present invention also extends to a
method of steering a charged particle beam such as an electron beam
in an additive layer manufacturing apparatus.
BACKGROUND TO THE INVENTION
[0002] Additive layer manufacturing is a maturing field. The
technique sees energy injected into a substrate medium to alter
parts of the substrate, for example to melt, fuse or harden the
substrate, so that it forms a layer of a product to be formed. New
substrate medium is added and the next layer formed, and so on. In
this way, a product may be formed layer-by-layer.
[0003] The substrate medium may be selected from many different
materials according to need. For example, plastics and metals are
commonly used as substrate media. Metal may be provided in powder
form from one or more hoppers and spread over a work table to form
a powder bed. As each layer is formed, the work table may be
lowered by the depth of the powder bed, and a new powder bed
deposited on the part-formed product.
[0004] The energy source is typically either a laser beam or an
electron beam. The present invention is primarily concerned with
electron beam sources, although extends to other types of charged
particle beams. Such electron beam sources are controlled using
electric and/or magnetic fields to steer or condition the electron
beam. Commonly, an electron beam is used as the energy source and
is steered using electromagnetic deflectors. These electromagnetic
deflectors allow the electron beam to be scanned across the
substrate medium, such that a pattern may be scanned or traced over
the substrate medium.
[0005] At the design stage, the product to be formed is mapped into
XYZ Cartesian coordinates. The product may then be "deconstructed"
into layers that will be formed successively to make the final
product. Each layer to be formed is described using XY Cartesian
coordinates (the Z coordinate is fixed for each layer). The
electromagnetic deflectors are used to scan the electron beam
according to a desired path defined using the XY coordinates as an
addressable grid. The shape to be formed in the powder bed for any
particular layer is formed by the beam as the beam follows a scan
pattern. The scan pattern usually comprises simple lines of
variable lengths, defined by vectors (for example linking a start
point and an end point for each straight line). These lines combine
to form the desired shape for the layer.
[0006] As the electron beam is scanned over the powder bed, energy
is deposited into the powder, raising its temperature. Exposure to
the electron beam is carefully controlled to ensure complete
melting of powder so that the powder particles within the top layer
fuse together and so that the powder particles within the top layer
also fuse with the previous layer thereby forming a solid product.
However, the energy deposited into the powder bed must be
controlled to prevent the generation of defects and ensure the
correct formation of the material microstructure.
[0007] A melt pool forms as the electron beam delivers its energy
and as the heat is conducted through the metal powder. Rather than
depositing all the required energy into a particular location of
the powder bed in one go by controlling the electron beam to dwell
at that location, the electron beam is usually scanned continuously
within the melt pool. Typically, the electron beam visits a
location multiple times. Each time the electron beam passes over a
location, the electron beam raises the temperature of that location
incrementally until the powder melts. It is also known to form
multiple separated melt pools rather than just a single melt pool
at any one time. Thus the electron beam may scan continuously
within a particular melt pool and then be deflected a relatively
long distance across the powder bed to another melt pool, and so
on.
[0008] Moving the electron beam from melt pool to melt pool means
that beam time and power is wasted as areas of the powder bed are
traversed repeatedly that do not require melting. Attempts to
address this problem to date have focussed on developing scan
algorithms to optimise the scan strategy and thermal control. For
example, much effort has been focussed on how best to transform
each layer's shape into a scan pattern that minimises transit of
areas of the powder bed that do not require melting. However, these
techniques are complex and geometry dependant. Generally, a bespoke
scan pattern must be developed for each product being made.
[0009] Also, current scan patterns place quite different
requirements on the electron beam control. Beam deflection speeds
as high as 8000 m/s have been reported for deflecting the electron
beam between melt pools, whereas slow deflection speeds of tens of
m/s are said to be required for achieving reasonable melt
characteristics when scanning within each melt pool. Satisfying the
different requirements for scanning within a melt pool as opposed
to scanning between melt pools presents a great challenge when
designing the electromagnetic deflectors.
SUMMARY OF THE INVENTION
[0010] From a first aspect, the present invention resides in a
method of forming a product using additive layer manufacture. The
method comprises forming the product as a series of layers, each
layer being formed by fusing powder deposited as a powder bed by
scanning the powder bed using a charged particle beam to form a
desired layer shape. For each layer, the powder is fused by melting
successive areas of the powder bed by scanning the charged particle
beam using a combination of a relatively long-range deflector and a
relatively short-range deflector wherein the relatively long-range
deflector deflects the charged particle beam over a larger
deflection angle than the short-range deflector.
[0011] The charged particle beam may be an electron beam, and the
charged particle beam is assumed to be an electron beam below.
However, it is to be understood that where the following describes
an electron beam, it could just as easily describe a charged
particle beam and the present invention is not restricted to
electron beams. The powder may be a metallic powder.
[0012] It has been realised that there are two aspects of beam
steering and control, and that these two aspects can be separated.
First, there is the long range deflection that allows the beam to
access all areas of the powder bed. This long range deflection may
be used to scan the electron beam between melt pools. Hence, the
relatively long-range deflector may be used to set a position of
the charged particle beam on the powder bed.
[0013] Second, there is the short-range deflection of the electron
beam used to achieve a desired scan pattern with the electron beam.
Hence, the relatively short-range deflector may be used to scan the
electron beam about the positions set by the relatively long-range
deflector. The precision of the relatively long- and short-range
deflectors may be the same. That is the relatively long- and
short-range deflectors may be able to set the position of the
electron beam on the powder bed with the same precision. Although
the precision may be the same, the relatively long-range deflector
is generally used to move the electron beam between positions with
greater step sizes than the relatively short-range deflector.
[0014] It has been realised that the long range deflection may be
effected by a first deflector, the relatively long-range deflector
(sometimes referred to herein as the "mainfield" deflector), and
the precise deflection may be effected as a small scale deflection
by a second deflector, the relatively small-range deflector
(sometimes referred to herein as the "subfield" deflector). The
relatively long-range deflector may deflect the electron beam over
the full extent of the powder bed, or at least over the majority of
the powder bed. On the other hand, the relatively short-range
deflector may deflect the electron beam over very much shorter
distances. For example, the relatively short-range deflector may
deflect the electron beam over only a fraction of the range of the
relatively long-range deflector, for instance 10% or less, or 1% or
less of the range. As the roles of the relatively long- and
short-range deflectors are separated, the design of the deflectors
may be optimised for their respective roles. For example, a slow
relatively long-range deflector may be used to effect the long
range movement of the electron beam, whereas a faster relatively
short-range deflector may be used to effect the very much shorter
range movement of the electron beam.
[0015] As much faster scan rates may be achieved for the electron
beam using the relatively short-range deflector, energy may be
deposited into the powder bed at a rate suitable for controlled
dispersal of thermal energy into the powder bed. This allows the
temperature of a location to be raised continuously as the electron
beam revisits that location, rather than see the temperature drop
between exposures to the electron beam. Consequently, in effect, an
area can be increased in temperature simultaneously rather than in
the staggered manner achieved by the prior art "line scan" method.
Thus, the present invention may be thought of as an "area scanner"
rather than as a "line scanner". That is, because the scan rate is
many times faster than the propagation of heat through the powder
bed, the area of the scan can be considered as if it had been
exposed to a single beam having a very specific shape. The area's
thermal profile can be tailored to compensate for the boundary
conditions of the area and ensure uniform melt.
[0016] Accordingly, the method of the present invention may
comprise using the relatively long-range deflector to set a base
position of the electron beam on the powder bed and using the
relatively small-range deflector to scan the electron beam around
the base position set by the relatively long-range deflector. The
method may comprise using the relatively small-range deflector to
scan the electron beam to trace a predetermined shape on the powder
bed. The predetermined shape, or "primitive" as it is otherwise
called herein, may be selected from a library of such predetermined
primitives. The primitives may comprise shapes such as squares,
rectangles, triangles and hexagons. Irregular primitives may be
used too. Also, primitives may be combined to form compound shapes
about the base position set by the relatively long-range deflector,
for example by combining two rectangles to form an L-shaped
compound shape.
[0017] The method may further comprise repeated steps of using the
relatively long-range deflector to set different base positions of
the electron beam on the powder bed and using the relatively
small-range deflector to scan the electron beam around each of the
different base positions set by the relatively long-range deflector
to trace a series of predetermined shapes on the powder bed,
wherein the predetermined shapes combine to create the desired
layer shape. For example, the predetermined shapes may be arranged
to abut or overlap to fill the desired layer shape. The majority of
the predetermined shapes may have a common size and shape and
tessellate to form part of the desired layer shape. Not all
predetermined shapes may be the same. For example, the majority of
a layer shape may be formed using the same predetermined shape, but
other shapes may be required to create the required edge to the
layer shape. For example, squares may be used to form the interior
of the layer shape, whereas the required edge may be approximated
using a series of triangles.
[0018] The method may comprise using the relatively long-range
deflector to set an array of base positions of the electron beam on
the powder bed, with the predetermined shapes scanned by the
electron beam about each base position tessellating without leaving
gaps therebetween thereby forming a part of the desired shape. For
example, squares may be used for the majority of predetermined
shapes, in which case the relatively long-range deflector may be
used to move the electron beam between a square array of positions
on the powder bed.
[0019] The relatively long-range deflector is preferably used to
scan the electron beam at a relatively slow speed and the
relatively short-range deflector is used to scan the electron beam
at a relatively fast speed. An electrostatic or electromagnetic
deflector may be used in either or both the relatively long-range
deflector and the relatively short-range deflector. Where one or
more electromagnetic deflectors are used, they will typically
comprise turns of coils of wire or other electrical
current-carrying material. As large deflection is required from the
relatively long-range deflector, a relatively large number of turns
may be used that may be driven by a relatively large current
(relative to the current passed through the relatively short-range
deflector). The large current and high number of turns in the
relatively long-range deflector means that it has a relatively high
inductance and hence a relatively slow slew rate (again, relative
to the relatively short-range deflector). In contrast, as the
relatively short-range deflector is required only to provide small
deflections of the electron beam, a relatively low number of turns
may be used and a relatively small current may be used to drive the
coils. This means that the relatively short-range deflector has a
relatively low inductance and hence a relatively high slew rate.
Consequently, the scan speeds achievable for the electron beam
while following the desired scan pattern within a melt pool is
increased relative to the prior art. By way of example only, the
relatively long-range deflector may be an electromagnetic deflector
comprising a Helmholtz coil with more than 25 turns per coil and
the relatively short-range deflector may be an electromagnetic
deflector comprising a Helmholtz coil with fewer than 5 turns per
coil.
[0020] Advantageously, the method may comprise using the relatively
short-range deflector to scan the electron beam to trace each
predetermined shape at a speed fast enough such that the
temperature of the powder bed at the start position of the scan is
substantially the same as temperature at the end position of the
scan when the electron beam has completed the scan to trace the
predetermined shape. The method may further comprise repeatedly
using the relatively small-range deflector to scan the electron
beam to trace the same predetermined shape at each base position on
the powder bed, thereby raising and maintaining the temperature of
the powder bed evenly within the predetermined shape. As noted
above, this method may be thought of as an "area scanner" rather
than as a "line scanner". That is, because the scan rate is many
times faster than the propagation of heat through the powder bed,
the area of the scan to create the predetermined shape can be
considered as if it had been exposed to a single beam having the
predetermined shape. Also, edge effects caused by the edge of the
melt pool losing more heat to surrounding unheated powder bed can
be addressed. Namely, the area's thermal profile can be tailored to
compensate for the boundary conditions of the area and ensure
uniform melt.
[0021] Movement of the electron beam using the relatively
long-range and short-range deflectors may be performed together or
separately.
[0022] When performed separately, the method may comprise setting
the relatively long-range deflector to position the electron beam
at the base position of the electron beam on the powder bed,
maintaining the setting of the relatively long-range deflector
while varying the setting of the relatively short-range deflector
to scan the electron beam around the base position set by the
relatively long-range deflector. The method may then comprise
further steps of changing the setting of the relatively long-range
deflector to position the electron beam at a different base
position of the electron beam on the powder bed, maintaining the
setting of the relatively long-range deflector while varying the
setting of the relatively short-range deflector to scan the
electron beam around the different base position set by the
relatively long-range deflector. Thus, a series of predetermined
shapes may be traced with the electron beam scanning about a base
position set by the relatively long-range deflector to form the
predetermined shape before the relatively long-range deflector is
used to move the electron beam onto the next base position for the
next shape to be traced, and so on.
[0023] When the relatively long-range and short-range deflectors
are used together the method may comprise varying the relatively
long-range deflector to scan the electron beam through a series of
base positions of the electron beam on the powder bed while varying
the setting of the relatively short-range deflector to scan the
electron beam around the base positions set by the relatively
long-range deflector. For example, the relatively long-range
deflector may be used to cause a slow scan of the electron beam
across the powder bed while the relatively short-range deflector
may be used to cause the electron beam to perform a fast scan about
the base position set by the relatively long-range deflector,
thereby forming the desired shape. Advantageously, as control of
the electron beam is split between two different deflectors, the
relatively short-range deflector that may be optimised for fast
scan speeds and high slew rates may be used to effect a very much
faster scan of the electron beam than if a single deflector was
used to effect both the long-range, low slew rate scan of the
electron beam across the powder bed as well as the short-range,
high slew rate, fast scan.
[0024] From a further aspect, the present invention resides in a
charged particle optical assembly for use in additive layer
manufacture. The assembly comprises a charged particle source; and
beam forming apparatus operable to form a beam of charged particles
from the charged particles provided by the charged particle source
that travels along a direction of propagation. The assembly further
comprises beam steering apparatus; wherein the beam steering
apparatus comprises a long-range deflector operable to deflect the
charged particle beam over a relatively large deflection angle and
a short-range deflector operable to deflect the charged particle
beam over only a relatively small deflection angle. The large and
small deflection angles are seen as long- and short-range movement
of the charged particle beam over the powder bed. As noted above,
the charged particle beam may be an electron beam, and the charged
particle beam is assumed to be an electron beam below. However, it
is to be understood that where the following describes an electron
beam, it could just as easily describe a charged particle beam and
the present invention is not restricted to electron beams.
[0025] The long-range deflector may be arranged to cause the
electron beam to deflect transversely to the direction of
propagation. The long-range deflector may comprise first and second
deflectors arranged to act orthogonally with respect to each other
and to the direction of propagation.
[0026] The short-range deflector may be arranged to cause the
electron beam to deflect transversely to the direction of
propagation. The short-range deflector may comprise first and
second deflectors arranged to act orthogonally with respect to each
other and to the direction of propagation. Where both the
long-range deflector and the short-range deflector comprise first
and second deflectors as described above, the first deflectors of
the long-range deflector and the short-range deflector may be
arranged to deflect the electron beam in a common direction. Also,
the second deflectors of the long-range deflector and the
short-range deflector may be arranged to deflect the electron beam
in a common direction. Hence, if the direction of propagation is
considered to define the Z axis of a Cartesian coordinate system,
the first deflectors of the long- and short-range deflectors may be
arranged to deflect the electron beam in the X direction and the
second deflectors may then be arranged to deflect the electron beam
in the Y direction.
[0027] Any, including all, of the deflectors may comprise
electromagnetic deflectors, for example Helmholtz coils. The
Helmholtz coils may comprise a coil of wire of other electrical
current carrying medium arranged to either side of the electron
beam path. Where Helmholtz coils are used for either or both the
first and second deflectors of the long-range deflector, each coil
may comprise 50 to 100 turns of wire. Where Helmholtz coils are
used for either or both the first and second deflectors of the
short-range deflector, each coil may comprise 1 to 5 turns of
wire.
[0028] Rather than using electromagnetic deflection, electrostatic
deflection may be used to steer the electron beam. Hence, the first
and second deflectors of both the long-range deflector and the
short-range deflector may comprise electrostatic deflectors that
are arranged to either side of the electron beam path.
[0029] Where electrons are used as the charged particles, the
charged particle optical assembly may comprise an electron source
acting as the charged particle source. The beam forming apparatus
may then form an electron beam, and the beam steering apparatus may
steer the electron beam.
[0030] The present invention also resides in an additive layer
manufacturing apparatus comprising any of the electron optical
assemblies described above, at least one hopper operable to
dispense powder, and a table positioned to receive the powder
dispensed by the at least one hopper in a volume defining a powder
bed for receiving the electron beam.
[0031] Optionally, the long-range deflector is operable to scan the
electron beam over at least half the area of the powder bed, for
example over at least 75%, 90% or 95% of the area of the powder
bed. Optionally, the short-range deflector is operable to scan the
electron beam over less than half the area of the powder bed, for
example over less than 25%, 10%, 5% or 1% of the area of the powder
bed.
[0032] The additive layer manufacturing may further comprise a
controller programmed to perform any of the methods described
above.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] In order that the invention can be more readily understood,
reference will now be made by way of example only, to the
accompanying drawings in which:
[0034] FIG. 1 shows additive layer manufacture apparatus with which
the present invention may be used;
[0035] FIG. 2 is a schematic representation of an electron source
and electromagnetic deflector assembly operable to provide an
electron beam and to scan the electron beam in accordance with
embodiments of the present invention, with FIG. 2a corresponding to
a side view and FIG. 2b corresponding to a view through line B-B of
FIG. 2a;
[0036] FIG. 3 is a schematic representation of a powder bed forming
a mainfield to be scanned by the electron beam, and how the
mainfield may be divided into subfields and primitives;
[0037] FIG. 4 is a schematic representation of a method of forming
a layer of a product during additive layer manufacture according to
an embodiment of the present invention;
[0038] FIG. 5 is a schematic representation of a powder bed, a
layer of another product to be formed, and another arrangement of
primitives that cover the layer of the product;
[0039] FIG. 6 is a schematic representation of a method of
generating a scan pattern for forming a product using additive
layer manufacture; and
[0040] FIG. 7 is a schematic representation of a method of
generating a scan pattern for forming a product using during
additive layer manufacture.
DETAILED DESCRIPTION OF EMBODIMENTS
[0041] FIG. 1 shows additive manufacturing apparatus 100 in which
embodiments of the present invention may be implemented. The
apparatus 100 is for additive layer manufacture of products from
metal powder using an electron beam.
[0042] To this end, the apparatus 100 comprises an electron optical
assembly 101 that forms, conditions and steers an electron beam
103, as will be described in more detail below. The apparatus 100
further comprises powder hoppers 121 containing metal powder 122
and a movable table 130. The hoppers 121 dispense powder so as to
lay down a thin layer of the powder on the table 130. Any number of
hoppers 121 may be used, and the two shown in FIG. 1 is but merely
an example. A mechanism such as a scraper or blade (not shown) may
be used to disperse the powder 122 evenly over the table 130. The
electron optical assembly 101 steers the electron beam 103 such
that the electron beam 103 is scanned over the powder bed 123 to
fuse the powder 122 and form a solid product 150.
[0043] After each layer of the product 150 has been formed, the
table 130 is lowered in the direction indicated by arrow 131. The
table 130 is lowered such that the top surface of the powder bed
123 is always formed at the same height relative to the electron
beam 103. The initial layer of the powder bed 123 may be deposited
to be thicker than the successive layers to minimise heat
conduction to the table 130 which may cause the powder 122 to fuse
with the table 130. Thus, a complete layer of unmelted powder 124
is left beneath the product 150 as it is formed.
[0044] Additive manufacture using electron beams is generally
performed under vacuum conditions, hence the apparatus 100
comprises an enclosing vacuum chamber 140. The vacuum within the
vacuum chamber 140 is created and maintained by a pumping system
144, such as any commonly available pumping system, for example a
turbomolecular pump backed by a roughing pump. The pumping system
144 may be controlled by the controller 110. As shown in FIG. 1,
the pumping system 144 may be used to evacuate the portion of the
vacuum chamber 140 housing the electron optical assembly 101. The
pressure in the vacuum chamber 140 may be in the range of
1.times.10.sup.-3 mbar to 1.times.10.sup.-6 mbar.
[0045] To this end, the apparatus 100 comprises an electron optical
assembly 101 including an electron source 102 for generating
electrons, lenses 220 for conditioning and forming an electron beam
103 from the emitted electrons and electromagnetic deflectors 240
and 250 for steering the electron beam 103. Operation of the
electron source 102 and deflectors 104 is controlled by a
controller 110 such as a suitably programmed computer.
[0046] FIGS. 2a and 2b show the electron optical assembly 101 in
more detail. The electron optical assembly 101 comprises an
electron source 102 for generating electrons, lenses 220 for
forming and conditioning an electron beam 103 from the emitted
electrons, and electromagnetic deflectors 240 and 250 for steering
the electron beam 103. Operation of the electron source 102, lenses
220 and deflectors 240, 250 is controlled by a controller 110 such
as a suitably programmed computer. Any conventional arrangement of
electron source 103 and lenses 220 may be used, and so will not be
described in detail here.
[0047] Essentially, the electron source 103 and lenses 220 deliver
a focussed electron beam 103 that is travelling along the central
axis 202 of the electron optical assembly 101.
[0048] Then, the electromagnetic deflectors 240 and 250 act to
steer the electron beam 103 across the powder bed 123 thereby
scanning the electron beam 103 according to a desired scan pattern.
The second of these deflectors 250 deflects the electron beam over
larger distances, and is referred to herein as the mainfield
deflector 250. This mainfield deflector 250 provides longer range
steering of the electron beam 103, and can steer the electron beam
103 across the full range (or "mainfield") of the powder bed 123.
The deflector that deflects the electron beam over smaller
distances is the subfield deflector 240 which effectively applies a
small dynamic disturbance to the electromagnetic field produced by
the mainfield deflector 250. This dynamic disturbance scans the
electron beam 103 about the position set by the mainfield deflector
250. Thus, use of the subfield deflector 240 allows the electron
beam 103 to be scanned through a small area or "subfield" of the
powder bed 123 about the base position set by the mainfield
deflector 250. Different subfields may be scanned by moving the
electron beam 103 to a different area of the powder bed 123 using
the mainfield deflector 250, as will be described in more detail
below.
[0049] Each of the deflectors 240, 250 described above may be
conventional electromagnetic deflectors comprising paired
current-carrying coils like Helmholtz coils or conventional
electrostatic deflectors comprising paired conductive plates set to
appropriate potentials. In either case, the deflectors are operated
by the controller 110 to provide the desired deflection, as is well
known in the art.
[0050] The mainfield deflectors 250 and the subfield deflectors 240
act transverse to the path of the electron beam 103 to steer the
electron beam 103 away from the central axis 202 (or to keep the
electron beam 103 travelling along the central axis 202). This
steering is separated into orthogonal components controlled by
separate deflectors. Accordingly, the deflectors 240, 250 are
provided in orthogonally disposed pairs to effect control of the
electron beam 103 in both X and Y coordinates, as shown in FIG. 2b.
Hence, there are four elements in each deflector. For example, FIG.
2b shows the mainfield deflector 250 comprising two Helmholtz
coils. A first pair of coils 250x are located to either side of the
electron beam 103 and separated in the X coordinate direction
thereby allowing the electron beam 103 to be steered in the X
direction. A second pair of coils 250y are located to either side
of the electron beam 103 and separated in the Y coordinate
direction thereby allowing the electron beam 103 to be steered in
the Y direction. This arrangement is repeated for the subfield
deflector 240.
[0051] The mainfield deflector 250 should be able to scan the
electron beam 103 over distances such as 0.1 m, 0.2 m, 0.3 m, 0.4
m, 0.5 m or even larger (in both X and Y coordinates, although the
X and Y deflection ranges need not be matched to provide square
mainfields such that rectangular mainfields may also arise). To
provide this relatively large deflection range, the mainfield
deflector 250 may be designed as Helmholtz coils with typically 50
to 100 turns of wire per coil carrying around 1 to 10 A of current,
and operating at frequencies around 100 kHz. Alternatively,
parallel electrode plates may be used in each axis across which a
variable voltage of magnitude of around .+-.5,000 V would be
applied in order to deflect the electron beam 103 in both the
positive and negative X and Y directions.
[0052] The subfield deflector 240 produces far finer deflection
ranges, for example distances of 0.001 m, 0.025 m, 0.005 m, 0.01 m
or 0.05 m (in both X and Y directions), although should be able to
drive the electron beam far more quickly than the mainfield
deflector 250. Accordingly, the subfield deflector 240 is likely to
be of a different design. For example, where a Helmholtz coil is
used, each coil will contain typically only a single turn or a few
turns of wire, carrying typically hundreds of milliamps up to 1 A
of current, and operating at frequencies around 25 MHz.
Alternatively, parallel electrode plates may be used in the
subfield deflector 240, as for the mainfield deflector 250, but
with a lower requirement on the applied voltage, such as .+-.200
V.
[0053] Further description will now be provided of the mainfield,
subfields and "primitives" with reference to FIG. 3. As noted
above, the range of deflection of the electron beam 103 provided by
the mainfield deflector 250 defines the size and shape of the
mainfield. Typically, the range of deflection in the X and Y
directions will be the same such that a square mainfield results.
FIG. 3 shows an example of a mainfield 300 extending 0.3 m by 0.3
m. The powder bed 123 may or may not correspond to this size. For
example, the powder bed may be slightly bigger to allow a margin
around the mainfield 300 such that powder from the edge of the
powder bed 123 does not fuse with the surrounding parts of the
apparatus 100.
[0054] As mentioned above, the electron beam 103 may be set to any
position in the mainfield using the mainfield deflector 250. The
electron beam 103 may then be scanned using the subfield deflector
240. The size of each subfield is set by the range of movement
provided by the subfield deflector 240. The range of deflection in
the X and Y directions is likely to be matched such that square
subfields result. Movement of the electron beam 103 using the
mainfield deflector 250 may be performed such that an array of
subfields arise that cover the entire mainfield, or a least the
part of the mainfield in which the current layer of the product 150
to be formed extends.
[0055] The detail from within the circle 305 of FIG. 3 shows how
the mainfield 300 may be divided into an addressable grid of
subfield squares 310 in this way. The subfields 310 are laid out
side by side in a patchwork pattern with no overlapping addressable
coordinates. Each subfield 310 measures 0.005 m by 0.005 m,
corresponding to the range of movement of the subfield deflector
240.
[0056] The mainfield deflector 250, which is capable of deflecting
the electron beam 103 to any of the subfield positions, places the
electron beam 103 into these subfields 310 in turn. For example,
the mainfield deflector 250 may position the electron beam 103 into
a base position at the lower right of each subfield 310 as shown at
320. Irrespective of where the start position of the electron beam
103 is within the subfield 310, the subfield deflector 240 then
scans the electron beam 103 to melt or raise the temperature of the
powder in the subfield 310.
[0057] The scan may see the electron beam 103 traverse all of the
area of the subfield 310, or the scan may see the electron beam 103
traverse only a part of the area of the subfield 310.
[0058] The shape traced by the electron beam 103 as it is scanned
within a subfield 310 is referred to herein as a "primitive". A
primitive may be smaller than a subfield or the same size as a
subfield.
[0059] Typically primitives corresponding to simple shapes such as
squares or triangles are used. In the example of FIG. 3, the
subfields 310 are square and so a square primitive is used to scan
the entire subfield. Moreover, primitives may be combined within a
subfield to form a compound shape. For example, two different sized
square primitives may be used to define an L-shape. Also, irregular
primitives may be used, for example to allow an irregular edge of a
product 150 to be formed. Examples of different shapes of
primitives are provided below with reference to FIGS. 4 to 6.
[0060] The primitives are formed by scanning the electron beam 103
using the subfield deflector 240 to trace out the desired shape.
For example, the subfield deflector 240 may cause the electron beam
103 to follow the raster pattern indicated by lines 330 of FIG. 3
until the electron beam 103 reaches an end position 340. In this
way, a square primitive filling the whole of the subfield 310 is
produced.
[0061] The spacing of the scan lines 330 may be set according to
the size of the electron beam 103, the speed of the scan rate and
the beam power, and other application specific parameters such as
the material, pattern density and neighbouring features. The fast
subfield deflector 240 allows power to be deposited in the powder
bed 123 at controlled rates that allows managed dispersal of the
thermal energy within the powder 122.
[0062] A further benefit is that the scanning required to expose
the powder bed 123 within the subfield 310 is performed by the
subfield deflector 240 which has a much faster and more accurate
scan capability than is attainable using prior art machines, whilst
the slower mainfield deflector 250 is simply used to position the
electron beam 103, very precisely, at the required subfield start
positions 320.
[0063] There is a substantial advantage to the fast subfield
deflector 240 allowing power to be deposited in the powder bed 123
at controlled rates suitable for the engineered dispersal of
thermal energy within the powder 122. Specifically, in effect, an
area scan can be performed that allows the temperature within the
whole area of the subfield 310 to be carefully controlled.
[0064] The fast scan rate allows an area to be scanned multiple
times such that the heat dispersal between visits is relatively
small, and therefore the area of the scan can be considered as if
it had been exposed to a single large electron beam 103 of a very
specific shape (i.e. the desired shape of the primitive or desired
compound shape, for example a square where the electron beam 103
scans all of the area within the exemplary subfields 310 shown in
FIG. 3). Thus, the electron beam 103 need no longer be considered
as having a simple Gaussian beam profile but rather an extended
shape capable of melting defined areas to form the desired
primitives and compound shapes.
[0065] Thus, as the electron beam 103 is moved from subfield 310 to
subfield 310, the layer of the product 150 being formed is
effectively scanned subfield by subfield. This results in the layer
of the product 150 effectively being formed by instantaneously
forming the primitives or compound shapes in each subfield 310 in
turn. The electron optical assembly 101 can therefore be envisioned
as a shape generator, allowing the primitives and compound shapes
to be "printed" to the powder bed 123. Thus, a layer to be formed
may be decomposed into these primitive shapes rather than being
decomposed into scan lines as is done in the prior art.
[0066] The power density incident on the powder bed 123 can be
readily controlled through the subfield deflector 240 and the area
the electron beam 103 traverses per unit time. The ability to
separate out the power density delivered to the powder bed 123 from
the current and energy of the electron beam 103 gives users another
degree of freedom in which to develop the process parameters for a
job. Moreover, the subfield scan rate and the high degree of
accuracy of the subfield deflector 240 allows fine control of the
melt pool formed in the subfield 310 and overcomes the need to run
multiple melt pools simultaneously. It should be noted too that the
lower inductance subfield deflector 240 will also have a much
faster positional settling time that the mainfield deflector
250.
[0067] The mainfield 300 of FIG. 3 describes the basic reference
grid for the electron gun assembly 101. The mainfield 300 is shown
to measure 0.3 m.times.0.3 m, although this will be a machine model
dependant parameter. The size of the subfield 310 can be selected
by the user and will be dependent on the product 150 being
manufactured. In the example of FIG. 3, the size of each subfield
310 has been selected as 0.005 m by 0.005 m, giving 360,000
subfields within the mainfield 300. The minimum pixel size for the
scan resolution (i.e. each individual addressable location for the
electron beam 103) is set by the user to a suitably small number
relative to the size of the subfield 310. For example, adjacent
pixel positions may be separated by 1.times.10.sup.-6 m in both X
and Y directions. This parameter may also be set by the user
according to the product 150 to be made. In this example, there are
250,000 addressable pixel positions per subfield 310, allowing
primitive shapes to be defined in fine detail, and
9.times.10.sup.10 addressable pixel positions in the mainfield
300.
[0068] The resolution and accuracy of the mainfield deflector 250
is set by the controller 110, for example by a digital to analog
converter (DAC) of the controller 110. As the mainfield deflector
250 must scan a larger area, it requires a higher bit count (circa
16 bits) compared to the subfield deflector 240 which covers a
smaller area (circa 12 bits).
[0069] As described above, the present invention allows layers of
the product 150 to be formed to be decomposed into primitive shapes
that combine to describe the two-dimensional pattern to be exposed.
This is in contrast to the prior art that uses a primitive shape
that represents only a simple line, with a start and end point and
a speed of traversal.
[0070] As will now be described with reference to FIGS. 4 to 6, a
library of "primitive" shapes may be used to describe different
shapes of products 150. For example, a layer may be decomposed into
primitive shapes corresponding to squares, rectangles, triangles,
hexagons and parallelograms, and any combinations thereof. Other
shapes are also possible.
[0071] FIG. 4 shows a layer 150 of a product to be formed in the
powder bed 123. The extent of the mainfield 300 is shown in the
figure, along with the division of the mainfield 300 into subfields
310 and also two examples of how the layer 151 may be decomposed
into primitives. In the first example, the layer 151 is formed
using square primitives 410a that correspond to the subfields 310.
That is, each subfield 310 is completely filled by driving the
subfield deflector 240 through its maximum range of deflection. The
detail of FIG. 4 also shows how the square subfields 310 and
primitives 410a may be arranged to fill a corner 152 to be formed
in the layer 151. That is, the locations of the electron beam 103
set by the mainfield deflector 250 may be chosen such that the
resulting subfields 310 align along the edges shown in the detail
of FIG. 4.
[0072] The second detail of FIG. 4 shows the layer 151 decomposed
into tessellating triangular primitives 410b. The size of the
triangles may be chosen so as to fill the shape of the layer 151 as
closely as possible to avoid the use of differently sized or
differently shaped primitives 410. In order to allow the electron
beam 103 to trace each primitive shape 410b, subfields 310 may be
defined that overlap. Alternatively, the triangles 410b may be
sized such that four adjacent triangles 410b fill a single square
subfield 310. Then, the triangles 410b may be traced in turn within
a single subfield 310, either consecutively with the mainfield
deflector 250 maintaining the electron beam position in the
subfield 310 or with intervals where the mainfield deflector 250 is
used to send the electron beam 103 to other subfields 310 before
returning to the subfield 310 to trace another triangle 410b.
[0073] As will be appreciated, not all shapes of layers 151 lend
themselves to decomposing into primitives 410 of the same shape.
FIG. 5 shows such an example. Here, the bulk of the layer 151 is
decomposed into square primitives 510a that fill each subfield 310.
However, the edges of the layer 151 require infilling with
irregularly shaped primitives that may vary from one to the next as
the edge of the layer 151 is followed. The detail of FIG. 5 shows a
curving corner 152 that is filled with a succession of
differently-shaped subfields 510b, c, d, e, f, etc., which have
curved boundaries.
[0074] Different strategies may be used to determine the order in
which to scan primitives 410, 510. For example, the electron beam
103 may be moved from one primitive 410, 510 to an adjacent
primitive 410, 510 and so on. Other arrangements are possible
though. For example, it may be advantageous not to scan a primitive
410, 510 until any adjacent primitives 410, 510 that have already
been scanned have returned to ambient temperature or close to
ambient temperature. The most efficient scanning strategy in order
to manage the thermal cool down properties of the material will be
decided by the application. The strategy allows for post melt
thermal management whereby the electron beam 103 can be used to
modify the thermal environment in order to create the desired
material properties.
[0075] FIG. 6 shows a method 600 of forming a layer 151 of a
product 150 according to an embodiment of the present invention.
The method 600 begins at 610. In this example, the electron beam
103 is started at step 610. This may not always be required. For
example, were a preceding layer 151 has just been formed, the
electron beam 103 may be kept switched on while a new powder bed
123 is deposited. In this case, the electron beam 103 may be
deflected away from the powder bed 123 to ensure that powder 122 is
not melted as it is being deposited. Where the electron beam 103 is
left switched on between layers, the electron current may be
decreased in which case it may not be necessary to deflect the
electron beam 103 away from the powder bed 123.
[0076] At step 620, the controller 110 uses the mainfield
deflectors 250 to move the electron beam 103 to an address within
the first subfield 310 to be processed. This address will be
specified in a scan pattern file that is made accessible to the
controller 110. As noted above, the scan rate of the electron beam
103 across the powder bed 123 will be relatively slow, compared to
the subfield deflection, as it is moved by the mainfield deflectors
250.
[0077] With the electron beam 103 in position within the first
specified subfield 310, the controller 110 uses the subfield
deflectors 240 to scan the electron beam 103 within the subfield
310 to trace and fill the desired primitive shape, as indicated at
step 630. As described above, the powder 122 within the primitive
410, 510 is effectively melted as a single area having the shape
defined by the primitive 410, 510. As also described above, the
primitive may be a compound shape formed of two or more primitives.
The primitives may be any of the primitive shapes, but simply
combined together to form the required compound shape. In this
embodiment, each primitive is traced for each step 630, i.e. the
electron beam 103 remains within a subfield 310 until all
primitives have been scanned and hence the compound shape is
complete.
[0078] At step 640, the controller 110 determines whether all
subfields within the layer 151 that require processing have been
processed. If not all subfields 310 that require processing have
been processed, the method loops back via path 645 to return to
step 620. At step 620, the controller 110 once again uses the
mainfield deflectors 250 to move the electron beam 103, this time
to the defined start position in the subfield 310 next specified in
the scan pattern. The method will then continue to step 630 which
sees that next subfield 310 processed by the electron beam 103 as
directed by the controller using the subfield deflectors 240. Step
640 sees another check as to whether all subfields 310 within the
current layer 151 that require scanning have been processed, with
multiple loops through steps 620 to 640 being performed until all
subfields 310 that require processing have been processed. At that
stage, the outcome at step 640 will be positive, such that the
method 600 exits to step 650.
[0079] In this example, at step 650, the electron beam 103 is
switched off. However, the electron beam 103 may alternatively be
reduced in current or left switched on but moved away so that
powder 122 may be deposited to form the powder bed 123 for the next
layer 151.
[0080] FIG. 7 shows a method 700 of generating a scan pattern for
forming a product 150 used during additive layer manufacture. The
method 700 begins at step 710 where a model of the product 150 is
obtained. This may comprise generating the model or may comprise
receiving or accessing a computer file that contains a description
of the model. In any event, step 710 sees a computer in possession
of a computer file that describes the size and shape of the product
150 to be formed. Such files, and the method to produce them, are
well known and so will not be described in any further detail.
[0081] At step 720, the computer decomposes the model of the
product 150 into layers 151 where each layer 151 represents a layer
151 through the product 150 that will be formed in a single support
table 130 position during the additive layer manufacture. Each
layer 151 will be defined by a Z coordinate, and the shape of the
layer 151 will be defined using X and Y coordinates.
[0082] Then, at step 730, the computer selects an unprocessed layer
151. This layer 151 may be the lowest layer 151. At step 740, the
computer decomposes the shape of the selected layer 151 into
subfields 310. With the layer 151 decomposed into subfields 310,
the computer then generates instructions to move the electron beam
103 between all the subfields 310 using the mainfield deflector
250.
[0083] Then, at step 750, the computer selects an unprocessed
subfield 310 and generates instructions to scan the primitive shape
or shapes for that subfield at step 760. These instructions
determine how the electron beam 103 is scanned within each subfield
310 as controlled using the subfield deflectors 240 to define the
desired primitive shape or shapes. As noted above, this may be a
compound shape formed of two or more primitives. This step 760 may
be performed by analysing the shape of the subfield 152 and finding
a suitably matched shape of primitive 410, 510 from a library of
primitive shapes.
[0084] At step 770, the computer determines whether all subfields
310 in the current layer 151 have been processed. If not, the
method 700 loops back along path 775 to step 750 where an
unprocessed subfield 310 is selected and subsequently processed
according to a further step 760, and the determination is again
made at step 770. This repeated loop continues until the
determination at step 770 indicates all subfields 310 in the
current layer 151 have been processed. As shown in FIG. 7, when a
positive determination is made at step 770, the method 700
continues to step 780 where another determination is made by the
computer.
[0085] Namely, a determination is made to ensure that instructions
are generated for all layers 151. As shown schematically in FIG. 7
at step 780, the computer in effect determines whether all layers
151 have been processed. If not, the method 700 loops back around
path 785 to return to step 730 to ensure that the next layer 151 is
selected at step 730 and processed at steps 740 to 770, and so on
until all the layers 151 have been processed and a set of scan
instructions has been generated that defines all layers 151 in the
product 150.
[0086] When all layers 151 have been processed in this way, the
method 700 continues to step 790 where the computer outputs a file
containing the complete scan instructions. This file may be saved
to memory, or may be sent to a controller like the controller 110
described above. In some embodiments, the controller 110 performs
the functions of the computer, i.e. the controller 110 may perform
the method 700 of FIG. 7 to generate the scan instructions and may
then execute the scan instructions by performing the method 600 of
FIG. 6. The scan instructions may be executed only after review and
approval by a user.
[0087] Those skilled in the art will appreciate that variations may
be made to the above embodiments without departing from the scope
of the invention that is defined by the appended claims.
[0088] For example, the embodiments described above all use an
electron beam 103 to melt the powder 122. However, other types of
charged particle beam may be used in the place of the electron beam
103.
[0089] In FIG. 7, the subfields 310 are defined at step 740 and
then the primitives 410, 510 are later defined at step 760. In this
case, the primitives 410, 510 must be made to fit in with the
pre-defined subfields 310. However, in other contemplated
embodiments, more flexibility may be achieved by combining steps
740 to 770. That is, for each layer 151 the layer 151 is decomposed
into subfields 310 and primitives 410, 510 at the same time. By
doing this, the placement of the subfields 310 may be determined to
optimise the selection of primitives 410, 510. For example, the
corner filling method described above with reference to the detail
shown in FIG. 4 may be implemented in this way.
* * * * *